1. Field of the Invention
This invention relates to a toner used in recording processes that utilize electrophotography, electrostatic recording or toner-jet recording. More particularly, this invention relates to a toner for developing electrostatic images, an image forming method making use of the toner, and an apparatus unit, used in copying machines, printers and facsimile machines in which a toner image is previously formed on an electrostatic latent image bearing member and thereafter transferred to a transfer medium to form an image.
2. Related Background Art
In toners used in electrophotography, electrostatic recording and toner-jet recording, it is commonly known to add small-particle-diameter inorganic fine particles to colored particles (toner particles) for the purpose of controlling the chargeability, fluidity and so forth of toner to attain good developing performance, cleaning performance and transfer performance.
However, toners to which such small-particle-diameter inorganic fine particles are added have been confirmed that toners having been used for a long time come into a state that the small-particle-diameter inorganic fine particles are buried in toner particle surfaces because of, e.g., the stress exerted thereon by carrier particles when used as two-component developers, the stress applied thereto from developer coating blade and developer feed roller when used as one-component developers, and the impact of toner particles against developing assembly inner walls and agitation blades and between toner particles.
In order to make such small-particle-diameter inorganic fine particles less buried in toner particles, it is effective to use large-particle-diameter inorganic fine particles in combination, as disclosed in Japanese Patent Application Laid-open No. 4-204751, No. 5-346682, No. 6-313980, No. 6-332235 and No. 7-92724.
The addition of large-particle-diameter inorganic fine particles brings about what is called the spacer effect, where the toner particle surfaces to which the small-particle-diameter inorganic fine particles having adhered can be prevented from coming into direct contact with the carrier, developer coating blade, developer feed roller, developing assembly inner walls and other toner particles to lessen the stress. This makes the small-particle-diameter inorganic fine particles less buried and enables achievement of a longer lifetime of the toner.
In order to make this spacer effect last, it is preferable to use silica as the large-particle-diameter inorganic fine particles. The reason therefor is as follows: The large-particle-diameter inorganic fine particles have a weaker electrostatic attraction to toner particle surfaces than the small-particle-diameter inorganic fine particles. Hence, the large-particle-diameter inorganic fine particles tend to become released from the toner particle surfaces to become smaller in quantity as a result of consumption during development, so that the spacer effect tends not to last. Here, the silica, which has a large charge quantity among inorganic fine particles and also is greatly attractable to toner particle surfaces, may become less released therefrom and can make the spacer effect last.
However, such toners to which the small-particle-diameter inorganic fine particles and large-particle-diameter silica have externally been added may come to have a too high chargeability in an environment of low humidity, i.e., tend to cause what is called the charge-up, showing an inferior environmental stability in some respect.
Meanwhile, as disclosed in Japanese Patent Application Laid-open No. 7-104501, a toner is proposed in which hydrophobic silica of 15 to 20 nm particle diameter, hydrophobic silica of 13 nm or smaller particle diameter and alumina are used as external additives. This toner can attain superior environmental properties in the two-component developers, which make use of carriers. However, in non-magnetic one-component developers, the hydrophobic silica of 15 to 20 nm may become released from toner particle surfaces to attain no sufficient performance with regard to the spacer effect, and the silica particles may acceleratedly become buried in toner particle surfaces, causing an increase in fog, faulty cleaning and a lowering of transfer efficiency. With regard to environmental properties, too, no sufficient performance can be obtained to cause a decrease in image density due to the charge-up and cause uneven images. This is considered due to the fact that the mechanical stress from the charging member blade is greater in non-magnetic one-component developers than in two-component developers.